Traditionally, oil and gas exploration is conducted using the seismic method. This geophysical technique provides a reasonable geometrical image of the subsurface structures outlying the possible location and shape of the hydrocarbon deposit. However, the seismic method may experience difficulties in discriminating between the deposits filled with water and the deposits filled with oil or gas. At the same time, the electrical properties of the water and hydrocarbon filled deposits may differ dramatically because oil and gas generally have very high resistivity (up to about 108 Ohm-m), while the water solutions in the rock formations are typically very conductive (about 1 Ohm-m and below).
Many existing electromagnetic technologies for marine oil and gas exploration are generally based on using either the magnetotelluric methods or placing the controlled source(s) in direct proximity to the target. There are very well known practical limitations of the marine controlled source electromagnetic (MCSEM) methods related to the limited depth of investigation. In order to increase the depth of the electromagnetic field penetration, one should typically use large transmitter/receiver offsets and, correspondingly, a very powerful transmitter. Both of these requirements may increase the technological difficulties as well as the cost of the MCSEM survey.
The magnetotelluric surveys are typically based on studying the electric and magnetic field variations at the sea-bottom due to the source in the ionosphere/magnetosphere. The magnetotelluric field, because of its regional nature, is practically uniform in the horizontal direction and may generate relatively weak vertical currents. As a result, the magnetotelluric field generally has very limited sensitivity and resolution with respect to thin horizontal resistive targets that are typical for the sea-bottom hydrocarbon deposits. In addition, the downward propagating magnetotelluric field may attenuate rapidly within the conductive layer of the sea water, which may limit the practical application of the magnetotelluric method in the deep-water surveys. Therefore a need exists for improved systems and methods for measuring sea-bed resistivity.